The present invention is related to combined electro-dynamic and friction brake systems and, in particular, to such brake systems for trains consisting of both powered and non-powered cars.
Combining electro-dynamic and friction braking is known as brake blending and is employed as a means of supplementing the electro-dynamic brake with friction braking as the electro-dynamic brake effectiveness diminishes during the course of a train stop. In this sense, the friction brake must be designed to provide 100% of the brake requirement in the absence of any electro-dynamic braking, with varying amounts of friction braking being provided in proportion to diminished electro-dynamic brake effect, so that the total brake effort of the combined electro-dynamic and friction braking satisfies the total braking requirement. Blending the electro-dynamic and friction brakes in this manner is known as direct brake blending.
Since the maximum electro-dynamic and friction brake levels are necessarily the same in direct brake blending systems, and the friction brake is limited by the allowable wheel temperature resulting from the friction between the brake shoes and wheel treads (where the friction brake is achieved through a conventional tread brake unit), it will be understood that this wheel temperature limiting factor also establishes the maximum level of electro-dynamic braking. Consequently, maximum utilization of the electro-dynamic brake capability, which is otherwise limited by the wheel/rail adhesion, is not achieved in direct brake blending systems, since generally the wheel temperature limitation establishes a greater restriction on the allowable brake effect than the wheel/rail adhesion limitation factor. This is of particular significance in train consists employing both power and non-power cars, where the power cars' electro-dynamic braking is a regenerative-type brake. The regenerative brake operates by dissipating the traction motor current generated during braking back into the power line via a catenary for use by other trains in the vicinity that are drawing current.
It will be apparent, therefore, that achieving less than maximum utilization of the regenerative capability of the electro-dynamic brake during retardation wastes available energy that could be put back into the power line, and further results in a high incidence of brake shoe wear due to excessive use of the pneumatic brake to supplement the electro-dynamic brake.